1. Field of the Invention
The present invention relates to a heat fixing unit having a temperature control means, for electrostatic recording apparatus or the like.
2. Description of the Prior Art
Generally, in the electrostatic recording apparatus or the like, toner image (powder image) is transferred from a charge receptor such as photoconductive member or the like onto a transfer medium such as paper, and the said toner image on the transfer medium is heated and fixed, and thus a copied image is obtained.
As a heat fixing unit in this electrostatic recording apparatus, a radiation type fixing unit that irradiates infrared rays, and a roller type fixing unit of contact heating system that utilizes thermal conduction are available, and the roller fixing unit has been widely used recently because the thermal efficiency thereof is high and consequently the electric power consumed is small; and also a speed-up of the recording apparatus is possible.
The aforesaid type of roller fixing unit is composed, in most cases, of a heat roller having a heat means (a heater lamp) at the center thereof, and of a pressure roller covered by an elastic substance such as silicon rubber or the like that makes pressure contact with said heating roller; a transfer medium to which toner images are transferred passes through these two rollers, and then said toner images in contact with the heating roller and are heated, to thus effect fixing. The driving device for both rollers of this roller fixing unit is generally disconnected for the period other than the fixing operation for the reason that it produces noise and for the additional reason of reducing electric power consumption. Furthermore if the pressure roller is held in contact with the heating roller for too long a period, material such as silicon rubber or the like that forms the surface of the pressure roller contacting the heating roller is sometimes deformed by heat. For such reason, both rollers are positioned with a clearance for the period when the transfer medium does not pass so that neither roller is deformed.
Therefore, in the roller fixing unit, there is a necessity for a so-called warming-up (WU) wherein a certain period of time is required from the start of energizing the heater up to the moment when the temperature sufficient for fixing is obtained. Immediately after this warming-up, the heat roller is hot but the pressure roller is in a state such that it is slightly warmed up only by the radiation of the heat roller during the warming-up. Consequently, if copying is made under such condition, the pressure roller contacts with the heating roller and the heat of the heating roller is used for the temperature rise on the pressure roller without being used for heating and fixing of toners on the transfer medium; thus the copying in the initial stage of operation is sometimes accompanied with an insufficient fixing.
Heretofore, as a countermeasure to solve this drawback, a method has been proposed to hold the established temperature of the heat roller high for a certain period of time immediately after the warming-up, thereby anticipating the amount of the heat to be carried away to the pressure roller, and a method to heat the pressure roller by arranging the heating means thereon separately from the heat roller and the like has also been known. However, in the method to hold the established temperature of the heating roller high, the heat resistance of the mechanism including the heat roller falls and the method to apply the separate heating means to the pressure roller has defects in that the structure thereof is complex and expensive.